I understand that, however I always ask for the Thai menu and can speak, read and write thai so maybe that's why I haven't encountered it. When it comes to haggling for something, my partner is much better but outside of haggling I've never encountered an issue, nor charged extra. Like I said, I'm aware it happens but it's not as widespread as it's made out to be.
Been living here over 4 years, and have visited here for much longer, never been scammed never been charged farang prices either. I'm not saying it doesn't happen, but it's nowhere near as common as some like to make out.
visa extension is 1900, re-entry is 1000 and at the airport it's + 100 if you don't have the required documents. So if you do it at the airport and don't have the documents required it's 1100
Best to call your airline and ask them. Regardless if it really comes down to needing it. Just book an onwards flight tickets online. Will cost you about $15. It's a legitimate ticket that you're reserving for a couple of days, if searched by anyone it will show in the system you have a ticket.
Without telling us what visa you have we can't really give you the proper information but based on what you have said I would guess you obtained METV (Multiple Entry Tourist Visa). This visa is valid for 6 months and allows you to enter Thailand multiple times during that 6 months, each time you enter you will be given an entry stamp which is valid for only 60 days, this however can be extended for another 30 days. Once your allotted time has expired, you will need to leave and re-enter Thailand. If it is indeed a METV. When done correctly you can get nearly 9 months of stay with an METV.
you can swap the physical sim but you can only use physical sim cards that are from the carrier. However you can pay about 30-40$ to unlock the phone so it's open to all sim cards.